Work has previously been carried out in investigating the electrical impedance of suspensions of certain bacteria. It has been established that the impedance consists primarily of resistive and capacitative components, though the measured conductivity and relative permittivity are not constant but functions of frequency. In a plot of relative permittivity against frequency, it is possible to identify alpha and beta dispersions which are believe to relate to the respective contributions of different membrane layers in the bacterium. Each membrane layer is believed to function as a resistance/capacitance network with the relative contribution of the various layers changing with frequency. The alpha dispersion has been associated with the outer membrane layers and the beta dispersion with the innermost layers.
The traditional approach to the measurement of impedance of bacterium suspensions involves manual bridge techniques. These are time consuming and require very careful attention to balancing and calibration. The procedures do not lend themselves to use in a bioreactor on a commercial scale where real time, continuous monitoring is required.
A number of commercially available impedance analysers have been suggested for use in investigating dielectric dispersion in suspension. The difficulty here is that the impedance analysers are constructed and organized to operate on networks of unknown but constant resistance, capacitance and inductance. Very careful further analysis is required, bearing in mind that the conductivity and relative permittivity of the suspension are known to be functions of frequency, before the output of a commercially available impedance analyser can provide useful information in this context.